1. Field of the Invention
This invention generally relates to a method of and arrangement for accurately measuring low capacitances and, more particularly, to the accurate measurement of capacitances less than 100 pF despite changing operating conditions between calibration and measurement modes of operation of a measurement circuit.
2. Description of Related Art
In radiosondes, capacitance detectors are used to measure various physical parameters, e.g. atmospheric pressure, temperature, humidity, etc. The magnitude of the measured capacitance indicates the value of the parameter. The capacitances of these detectors are relatively low, from a few pF to a few dozen pF, or, at the maximum, about 100 pF. The measurement of such low capacitances is not always accurate owing to stray capacitance, variations in supply voltage, changes in the external temperature, and other disturbances.
Reference can be had to U.S. Pat. Nos. 4,295,090 and 4,295,091, as well as U.S. patent application Ser. Nos. 739,325; 739,326 and 739,314 for details of known measurement circuits for measuring low capacitances by connecting a measurement capacitor, and one or more reference capacitors, one at a time, to an oscillator whose output frequency is proportional to the connected capacitor.
Although generally satisfactory for their intended purpose, the known measurement circuits have a measurement response characteristic which varies as a function of different operating conditions, thereby leading to unacceptable inaccurate measurements. For example, the measurement circuits of the aforementioned patents and applications are initially operated in a calibration mode of operation, and thereupon, after a predetermined time, in a measurement mode of operation. If the temperature of the measurement circuit and/or of the reference and/or measurement capacitors is different during the calibration and subsequent measurement modes of operation, the accuracy of measurement is compromised. If the loading impedance of the oscillator of the measurement circuit is different during the calibration and subsequent measurement modes of operation, then, again, measurement inaccuracies result.